The present invention relates to a camera and a signal processing method thereof. More particularly, the present invention relates to a camera for canceling a dark signal component resulted from an image pickup device such as a CCD (Charge Coupled Device), and relates to a signal processing method of the camera.
In recent years, mobile tools such as electronic still cameras and notebook personal computers have become widespread as the development of the semiconductor technology has surged forward. Especially, the electronic still camera picks up an image of a subject incident through an objective lens to generate electric signals by using an image sensor, and based on the electric signals displays the image on a liquid crystal display, or stores the image data in a nonvolatile semiconductor memory card. Further, the image data can be captured into a personal computer and subjected to an image processing. Therefore, the electronic still cameras have become widespread rapidly.
Meanwhile, it is known that the CCD which is mounted in the above-described electronic still camera has so-called dark output (dark voltage) characteristics in which output voltage is generated even when incident light is intercepted, and with this dark output, a very small current called dark current is generated.
This dark voltage in the CCD has temperature dependence in which dark voltage is approximately doubled if environmental temperature is varied (increased) about 8° C. The dark voltage has the exposure time (electric charge accumulation period) dependence indicative of tendency that the dark voltage is increased as the exposure time is longer under the same temperature condition.
The dark voltage of the CCD becomes noise component and as a result, this is the factor that affects the image data and deteriorates image quality.
As one of techniques for solving this problem, there is a known technique in which image data (dark output component) generated when a mechanical shutter is closed is subtracted from image data (subject image data including dark output component) generated when the mechanical shutter is opened, thereby canceling the noise component from the image data.
However, according to this technique, there is a problem that if the subtraction is carried out using image data which have been subjected to gamma-correction processing having nonlinear characteristics or saturated image data, or if the subtraction is carried out even when the level of the dark output component is low, excellent image data can not be obtained.